U.S. Pat. No. 4,202,933, to Reiser and Landau, describes that operation of fuel cell power plants at lower power levels causes high cell voltages which result in corrosion of catalysts and catalyst-support-material. This causes severe performance decay at low power operation. In vehicles for which the demand for power frequently is zero, recurrent, low power idling of the power plant will cause increasing performance decay over time.
The aforesaid patent further points out that just reducing the air input to the cathodes results in a severe current density maldistribution in each of the fuel cells. That is, the current density near the air inlets of the fuel cells is very high but the current density near air outlets is very low. This results in excessive heating at the air inlet regions of the fuel cells which can cause additional performance loss and possible fracture of fuel cell components.
In the aforesaid patent, cathode exhaust is recycled to the cathode air inlets in order to reduce power generated while at the same time controlling, at least to some extent, the voltage which is reached in the individual cells. This reduces current density maldistribution and corrosion. In the aforesaid patent, the desired power output is provided for comparison with actual power output; if the actual power is greater than the desired power, cathode recycle is used to cause a voltage drop; power conditioning apparatus reduces the actual power output. This can continue until either the desired power output level or the critical voltage is reached.
The problem with that method is that as much as 20 kilowatts may have to be generated in order to keep the voltage below the critical level. Although that power may be used for charging batteries or for running auxiliary equipment, it will in many instances simply have to be dissipated as heat, which can be a very difficult problem, particularly in a vehicle.
U.S. Pat. No. 4,859,545, to Scheffler and Vartanian, utilizes an algorithm involving oxygen concentration at the cathode exhaust, oxygen molar consumption rates at the present current output, and gas flow into the cathode, to control air utilization to ensure against individual cell oxygen starvation. The air inlet valve is adjusted upwardly or downwardly to achieve the desired air utilization. The manner of dealing with the output power is not disclosed.